Study On The Elastocaloric Effect And Shape Memory Properties Of NiTi Alloys Fabricated By Selective Laser Melting

NiTi memory alloys have been used in aerospace,intelligent drive,biomedical and other fields due to their excellent functional properties.However,due to its poor machinability,it is difficult to fabricate devices with complex structures using traditional methods,which severely limits its application range.The high degree of design freedom and complex component forming capability of selective laser melting（SLM）additive manufacturing technology can provide broad prospects for the multi-scenario application of Ni Ti alloys.However,in the process of SLM forming parts,due to the complex interaction between the dynamic melt flow formed by the high-energy laser beam and the powder and ambient gas,high-density dislocations,hole defects,residual stress,etc.are inevitably introduced,which seriously damages the performance of Ni Ti alloys.Super elastic properties and shape memory function.At the same time,compared with commercial Ni Ti alloys prepared by traditional methods,Ni Ti alloys prepared by SLM have coarse grains（several to tens of microns）and low yield strength,resulting in small superelastic strain.In addition,the Ni Ti alloy components prepared by SLM in the field of refrigeration are faced with problems such as poor superelastic cycle stability and low adiabatic temperature.In order to solve the above problems,the following researches are carried out in this thesis:The process parameters of different laser power,scanning speed and scanning distance were designed by orthogonal experiment method,and the influence law of process parameters and energy density on the forming quality of SLM-Ni_51.5Ti_48.5（at.%）alloy was explored;Differential scanning calorimetry（DSC）and penetrating Mo target high-throughput XRD diffractometer revealed the phase transformation behavior and phase composition of Ni_51.5Ti_48.5（at.%）alloy with energy density;in order to obtain accurate Adiabatic temperature changeΔTad,the effect of strain rate on the elastic-thermal properties of Ni_51.5Ti_48.5（at.%）alloy was explored,and the superelasticity of compressive strain on the directly prepared Ni_51.5Ti_48.5（at.%）alloy was revealed The law of influence of energy and elastic heat.At room temperature,the directly prepared Ni_51.5Ti_48.5（at.%）alloy can obtain a superelastic recovery strain of 5.5%-7%under a compressive strain of 10%,and the maximum adiabatic temperature change during unloading can reach-11.3℃,after 100 cycles,the adiabatic temperature change during unloading remains at about-8℃,which is higher than that reported.At the same time,the compressive recoverable strains of Ni_51.5Ti_48.5（at.%）alloys with different energy densities at a strain of 10%were increased to more than 8%by the combination of low-temperature solid solution and low-temperature aging.The elastic thermal properties of the samples were not significantly improved after treatment.In order to obtain SLM-NiTi alloys with excellent shape memory function,Ni_50.2Ti_49.8（at.%）alloys with good density were prepared by sequentially changing the scanning distance,scanning speed and laser power.The variation law of density,grain size and defect of Ni_50.2Ti_49.8（at.%）alloy with process parameters was revealed;The phase transformation behavior was characterized,and the change law of the phase transformation behavior with the process parameters was revealed.Through the mechanical property test,the influence law of the process parameters on the mechanical properties and shape memory function of the Ni_50.2Ti_49.8（at.%）alloy was revealed.The maximum density of Ni_50.2Ti_49.8（at.%）alloy formed by SLM can reach 99.98%,and it has uniform and fine grains around 2μm,and can obtain fracture strength higher than 800Mpa and 18%at room temperature The fracture strain of 81%was obtained for the first time in this work under the condition of room temperature tensile prevariance of 8%among Ni Ti alloys prepared by existing SLM.